Abstract: Test equipment does not always have to be expensive.
A simple horn antenna made from an asparagus can is described and its use
outlined. This device has been successfully used for debugging emissions
problems in new designs.

Discussion: As equipment designs increase in speed, emissions above
1 GHz have become more important. There is a multitude of pre-compliance
tests and simple equipment that have worked well below 1 GHz for years, but
more of this type of equipment is needed for the higher frequency ranges.
Radiation above 1 GHz can occur in narrow beams and can be difficult to find
without expensive antennae. Figure 1 shows a simple horn antenna constructed
from an asparagus can which turns out to have good dimensions for such a
troubleshooting antenna. Its length of about two to three times its diameter helps give the antenna good directivity.

Figure 2 shows an end view of the antenna. The monopole length is the radius
of the can and it is placed that distance from the back of the can. Copper
tape is used to form a slit in the end to make the antenna more sensitive
to the polarization of the received wave. The useful frequency range is that for
which the can radius is about one quarter wavelength. Below that frequency,
sensitivity falls off (waveguide beyond cutoff) and well above that frequency
multiple modes of transmission can occur in the can reducing its usefulness
with respect to polarization and directivity.

Figure 2. Can Horn Antenna - End View

A set of cans of varying sizes (tomato paste, asparagus, etc.) can be used to cover a wide range of frequencies.

Use the can horn antenna by positioning it a few feet from an equipment
under test (EUT). Keeping it generally pointed at the EUT, move the can around
the surface of an imaginary sphere enclosing the EUT, rotating the can to
pickup different polarizations of an emitted wave. For large EUTs, one may
want to point the antenna at various parts of the EUT from each position
on the sphere. When a signal is picked up on a receiver or spectrum
analyzer, one can move the antenna closer, following the signal. Often the
source of the emission can be resolved this way.

Note that radiated emissions at these elevated frequencies can "bounce and scatter" in
complex random interactions to the point of making the specific source
difficult to locate. In this situation, the "horn" antenna can
be placed directly above the suspected product area, such as gaps, seams,
or gap arrays (such as finger-stock). By sliding the aperture around in
contact with the product surface, the emissions can be directed into the
antenna itself, increasing the resolution of the source identification.

While not exactly a "calibrated" antenna, it can find problems in the development lab simply and inexpensively.

The idea for this antenna was contributed by W.Michael King. It is based on a previous urgent need
for a probe to detect problems at these frequencies when
he was working in a very remote location in the Andes Mountains - in 1966! Michael has been
known to say "I looked at the
concept of the "feed horn" in our X-band parabola, and couldn't
resist the temptation to call this a "food horn." His website is
http://www.systemsemc.com.